One of the most thorough studies of its kind has concluded that smoking too much reefer as a teenager is linked to significant cognitive decline. More frightening still: the effects could be permanent. Here's everything you need to know about the study's findings.

Here's the gist: The study followed 1,037 New Zealanders from birth through age 38, monitoring their IQs and marijuana usage throughout. IQ tests were administered first at age 13 (before the test subjects had started smoking pot), and again at 38, "after a a pattern of persistent cannabis use had developed."

The findings reveal that test subjects who smoked heavily throughout adolescence suffered an average drop of eight points in their IQ scores. According to Madeline H. Meier, first author of the study, this impairment was most prevalent among adolescent-onset cannabis users:

Adolescent onset users, who diagnosed with cannabis dependence before age 18, tended to become more persistent users, but... after equating adolescent- and adult-onset cannabis users on total number of cannabis-dependence diagnoses, adolescent-onset users showed greater IQ decline than adult-onset cannabis users. In fact, adult-onset cannabis users did not appear to experience IQ decline as a function of persistent cannabis use. [emphasis added]

Effects of persistent, early-onset cannabis use also appear to be global, meaning they were detectable across a variety of mental functions ranging from processing speed to verbal comprehension and working memory. Test subjects also nominated people "who knew them well" to complete questionnaires relating to problems with their attention and memory. The researchers write that "informants reported observing signiﬁcantly more attention and memory problems among those with more persistent cannabis dependence." Perhaps most unsettling of all, however, is the observation that the cognitive decline appears to be permanent; "cessation of cannabis use," note the researchers, "did not fully restore neuropsychological functioning among adolescent-onset cannabis users." (A glimmer of hope: previous studies indicate that cognitive recovery is possible after quitting weed.)

There are several things that make this study noteworthy. For instance, following test subjects over the course of four decades allowed Meier and her colleagues to gauge participants' cognitive abilities before they started smoking weed. The researchers claim this allowed them to rule out pre-existing neuropsychological impairment as an explanation for the link between early-onset marijuana use and subsequent cognitive decline. In other words: if a causative relationship exists between weed and cognitive impairment, it appears to be the pot-use that's begetting the impairment, not the other way around.

It's important to point out, by the way, that at no point do the researchers say that early-onset marijuana use causes cognitive decline. As Meier and her colleagues explicitly state: "our data
cannot deﬁnitively attest to whether this association is causal," nor can it "reveal the
mechanism underlying the association between persistent cannabis dependence and neuropsychological decline." In other words, we're still dealing in terms of correlation, here. Having said that, the study does go a long way in ruling out a number of alternative explanations that have confounded the results of similar studies linking cannabis and cognitive decline in the past, including hard-drug and alcohol dependence, and the years of education received by users versus non-users.

The biggest question raised by this study — how much pot is too much? — points to its limitations. Reports of drug-use in the study were self-reported, which, unlike biological assays (a urine test, for example), are inherently subjective.

But the question of "how much" is arguable less important than "when," or "what age?" The fact that cognitive impairment was detected in adolescent-onset smokers and not adult-onset smokers is highly suggestive that weed has a neurotoxic effect on the developing adolescent brain (what Meier and her colleagues refer to as the "developmental vulnerability hypothesis"), a conclusion that gels with a number of previousstudies.

"Puberty is a period of critical brain development, characterized by neuronal maturation and rearrangement processes and the maturation of neurotransmitter systems (e.g., the endogenous cannabinoid system)," conclude the researchers, "making the pubertal brain vulnerable to toxic insult." They continue:

Prevention and policy efforts should focus on delivering to the public the message that cannabis use during adolescence can have harmful effects on neuropsychological functioning, delaying the onset of cannabis use at least until adulthood, and encouraging cessation of cannabis use particularly for those who began using cannabis in adolescence.